1. Field of the Invention
The present general inventive concept relates to an OFDM (orthogonal frequency division multiplexing) receiving circuit, and more particularly to an OFDM receiving circuit having a plurality of demodulation paths to improve a performance of an oversampling ADC.
2. Background of the Related Art
An OFDM is a type of a multi-carrier modulation, where a symbol array inputted in series is converted into a parallel form by N block unit, then each element symbol is modulated into a sub-carrier having a mutual orthogonality, and then the sub-carriers are added for a transmission. The OFDM is robust to a multiple path fading occurring in a wireless communication environment and is capable of a high speed data transmission. Accordingly, use of the OFDM is increasing. The OFDM is used as a transmission method of a wireless LAN (e.g., IEEE 802.11a), Wibro (wireless broadband), WiMAX (World Interoperability for Microwave Access) and a terrestrial DMB (Digital Multimedia Broadcasting).
FIG. 1 is a diagram illustrating a conventional OFDM receiving circuit. As shown in FIG. 1, the conventional OFDM receiving circuit includes a low noise amplifier 11, a down-conversion mixer 13, a variable gain amplifier 15, a filter 17, an ADC (analog-to-digital converter) 19, a demodulator 21 and a local oscillator 23.
The conventional OFDM receiving circuit shown in FIG. 1 has a single demodulation path similar to other receiving circuits such as a CDMA receiving circuit. The single demodulation path refers to the single filter 17 (although the filter 17 is divided into an I channel filter and a Q channel filter, the I channel filter and the Q channel filter are regarded as the single filter 17 for convenience), the single ADC 19 (although the ADC 19 is divided into an I channel ADC and a Q channel ADC, the I channel ADC and the Q channel ADC are regarded as the single ADC 19 for convenience) and the single demodulator 21 for an OFDM signal band. For example, in the Wibro standard, the single filter 17, the single ADC 19 and the single demodulator 21 are used for an OFDM signal having a bandwidth of 8.75 MHz including 841 sub-carriers. In this case, the nyquist rate ADC 19 has a sampling frequency of 10 MHz.
FIG. 2 is a diagram illustrating a received signal after passing through the down-conversion mixer 13 of the conventional OFDM receiving circuit of FIG. 1. In FIG. 2, a band between +FB and −FB corresponds to the OFDM signal band. In accordance with the Wibro standard, +FB corresponds to 4.375 MHz. In addition, a frequency response of the filter 17 when the nyquist rate ADC 19 is used is shown in FIG. 2.
On the other hand, it is generally known that a performance of the ADC can be improved when an oversampling is carried out. When the oversampling is carried out, an SNR (signal-to-noise ratio) within the frequency band (between +FB and −FB) may be expressed as equation 1.SNRdB=10 log(12*PS/VFS2)+6N+10 log(OSR)  [Equation 1]where PS denotes a power of an input signal, VFS denotes a dynamic range of an entirety of the input signal, N denotes a number of quantization bits, and OSR denotes an oversampling ratio wherein OSR corresponds to (FS/2)/FB when a sampling frequency is FS.
As expressed in equation 3, when OSR is doubled, a performance improvement of 3 dB is obtained, which equivalently provides an increase of 0.5 bits in a resolution. That is, a principle of the oversampling ADC is that increasing the sampling frequency, which equivalently increases the number of bits obtained by the ADC, reduces a power of a quantization noise.
However, since the OFDM compliant to the Wibro standard has the signal band of 8.75 MHz, a sampling frequency much higher than that of the CDMA compliant to IS95 standard having a signal band of 1.25 MHz is required in order to carry out the oversampling. For instance, when the OSR is 8, the sampling frequency FS of the ADC should be 70 MHz (=8*8.75 MHz). Moreover, in order to increase the OSR for an improvement of the system performance, the sampling frequency FS increasing proportional to the OSR is required. However, when the sampling frequency increases, an aperture error and an error by a clock jitter increase, which limits the increase of the sampling frequency. Therefore, it is difficult to apply the oversampling ADC to the OFDM receiving circuit of FIG. 1. Even when the oversampling ADC is applied, it is difficult to increase the OSR (e.g., to have a value of more than 8).
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.